1. Technical Field
This disclosure relates to a flame-retardant polyester film having an excellent flame retardancy and processed products including the same.
2. Description of the Related Art
Polyester films are used as magnetic recording materials, electrical insulation materials, capacitor materials, packaging materials, and building materials because of their mechanical characteristics, electrical characteristics, and the like. Furthermore, polyester films are used as various industrial materials for photographic applications, graphic applications, thermal transfer applications, and the like. However, there are problems in that polyester films are softened or melt by heat, and are flammable. In particular, when polyester films are used as electrical insulation materials for applications related to adhesive tapes, flexible printed circuits, membrane switches, film heaters, or flat cables, or are used as building materials, since there is a danger that a fire breaks out, requirements for further improvement in the flame retardancy of polyester films have become intensified.
With respect to technologies for improving the flame retardancy of polyester film, a method in which a bromine-based, phosphorous-based, inorganic, or other flame retardant is incorporated into a polyester film or a method in which a halogen-containing component or a phosphorous-containing component is subjected to copolymerization has been proposed (for example, Japanese Unexamined Patent Application. Publication No. 10-278206).
However in that invention, there are problems in that, for example, repeated exposure to flame causes enlargement of burning and, therefore, the flame retardancy is inadequate. With respect to these technologies, since the flame retardant is added to the polyester film, or the halogen—containing component or the phosphorous-containing component is included in the polyester by copolymerization, the mechanical characteristics inherent in the polyester film are degraded. In addition, the halogen compounds may adversely affect the environment. For example, it is feared that dioxins and the like are generated from the halogen compounds depending on the burning conditions. Furthermore, there is a problem in that generated gases may cause contamination in the process.
Another method has been proposed, in which a resin, e.g., polyamic acid, is laminated on a polyester film and, thereby, heat resistance and flame retardancy are imparted (for example, Japanese Unexamined Patent Application Publication No. 2002-172747).
In that invention, there is a problem in that repeated exposure to flame causes enlargement of burning.
With respect to the technology for improving the flame retardancy of adhesive tapes or membrane switches prepared by processing polyester films, a method has been proposed, in which a flame retardant is added to a bonding agent layer provided between layers in a processed product.
In that invention, the amount of the flame retardant added to the bonding agent layer must be increased in order to attain adequate flame retardancy. Consequently, when these processed product are further processed for end uses, there are problems in that, for example, the contaminations in the process occur due to seepage and the like of the flame retardant, and the discoloration of the processed product occur due to decomposition of the flame retardant during a heat treatment. Likewise, with respect to the flexible printed circuits, the film heaters, and the flat cables prepared by processing the polyester films, a method is known, in which a flame retardant, e.g., a halogen compound, is added to a bonding-agent layer or an adhesive layer provided between layers in a processed product, but there are problems of, for example, seepage of the flame retardant added to the bonding-agent layer or the adhesive layer during processing because the amount of addition must be increased and the like.
It would therefore be advantageous to provide a flame-retardant polyester film having an excellent flame retardancy, and adhesive tapes, flexible printed circuits, a membrane switch, a film heater, and a flat cable, each including the flame-retardant polyester film.